Shopsmith Forums

I started this thread because I found myself posting in the "Power Pro" thread, shame on me!

Are all DC motors inherently variable speed?

Is the method for changing speeds as simple as changing the DC voltage to the motor?

Dusty:

I've worked for two electric motor manufacturers, but I don't claim to be the ultimate expert. DC motors vary widely in types, from high HP industrial motors to sub fractional permanent magnet motors.

For an application such as a home woodworking tool, let's take an example of a 1 HP DC motor with a base speed of 1800 rpm. A simple controller would rectify incoming AC and apply variable voltage to the rotor via the brushes. By turning a rheostat you can vary the speed, for most applications this would be downward. The motors are rated constant torque, which means at 900 rpm you only have 1/2 HP. This ratio continues proportionally as you go further down in speed. Thus, this type of motor is not going to work very well with a 3" forstner bit at 100 rpm. If the controller allows higher speeds, you would have constant HP above the base speed, and the torque would lower proportionally as you increase speed.

The DVR motor on the Power Pro, on the other hand is able to "sense" the load at any speed and provide the necessary torque. In our application, torque is equally important to variable speed.

Our time tested variable pitch pulley headstocks are powered by a two pole motor that has a full load speed of around 3500 rpm. As we lower the speed, the mechanical nature of the beast actually gives us increased torque, constant HP - up to a point! The belts, pulleys, bearings, and shafts can only provide so much torque before failing. This may be one of the reasons why the gilmer belt was replaced with the poly V belt. On the other hand, when we go to the highest speed, 5200 rpm, the rated torque output cannot exceed the 3500 rpm value. The AC induction motor, with its relatively flat speed torque curve, is capable of producing more than rated torque at this speed, but cannot do it continuously without overheating the motor.

Clear as mud?? Sorry.

I believe the answer basically is yes. You vary the voltage and the speed changes, but without overloading the motor with too much voltage or applying too little voltage and causing the motor to run so slow it overheats.

dusty wrote:I started this thread because I found myself posting in the "Power Pro" thread, shame on me!

Are all DC motors inherently variable speed?

Is the method for changing speeds as simple as changing the DC voltage to the motor?

Yes, DC motors are variable speed motors. The speed is varied by changing the voltage and/or ampere to the motor. DC motors uses brushes to connect the power to the armature. However, you need some type of DC controller to prevent excessive current and/or voltage which can overload the DC PM motor and kill the magnetic fields in the motor.

You can find more information on DC controllers at http://www.kbelectronics.com/ I use the KBIC and KBMM DC controllers from this company.

Following is from Skip Campbell, 10ER Guru Forum response:
One more thing about the dc motor torque issue. With a good dc controller the drop off in torque is not linear with the rpm. You will get great torque until you get down to around 10% of max rpm. The controllers have feedback circuitry and sense the motor rpm. As the load increases the controller jacks up the voltage (and correspondingly the current) to keep the motor going the same speed. It is only at the very bottom of the range where the current limiting circuits in the controller prevent it from maintaining full torque. Therefore, with a 1750 rpm dc motor and a good controller you will get good torque down to around 175 rpm. These are very general numbers and would vary from motor to motor. If you step down by .5 the rpm with pulleys the torque would be doubled so you torque should be good down to around 70 rpm.